The disc centrifuge photosedimentometer has proved to be an excellent instrument for providing particle size and particle size distribution information to pigment and latex manufacturers and formulators. In use, a disc centrifuge photosedimentometer operates by forcing particles (generally of less than about 2.mu. in size) under high centrifugal force radially outwardly through a spin fluid or medium. Here they segregate into sizes, the larger traversing the medium more quickly and the smaller taking a longer time. In general, bands of particles, of approximately the same size are created in the medium and can be measured optically with the aid of light or other suitable radiation to which the particles are opaque traversing the spin medium in an axial direction. The bands so created intercept the radiation and can be analyzed and the data obtained converted to particle size and/or distribution curves.
A particularly satisfactory method for photosedimentometric analysis is that described and claimed in U.S. Pat. No. 3,475,968 dated Nov. 4, 1969. U.S. Pat. No. 4,311,039 dated Jan. 19, 1982 describes and claims an apparatus which is especially effective in practicing the method of the aforesaid U.S. Pat. No. 3,475,968. The apparatus of the aforesaid U.S. Pat. No. 4,311,039, with or without modification, can be used most satisfactorily in practicing the present method. Thus, the disclosure of U.S. Pat. No. 4,311,039 is incorporated herein by reference.
The homogeneous, the line start and the buffered line start methods are well known. The latter two systems involve preparation of spin fluids or spin media in situ, i.e., during high speed rotation. Reference may be had to Chemical Analysis, "Direct Characterization of Fine Particles" by B. H. Kaye, Volume 61, John Wiley & Sons, 1981, pages 189-226; Schnelle Dichtegradienten - Zentrifugation dispergierter Teilchen, Lange, Colloid & Polymer Sci., 258, p. 1077-1085 (1980) CODEN CPMSS); "Zur Sedimentationsanalyse wassriger Kunststoffdispersionen mit der Scheibenzentrifuge" Langer, Colloid and Polymer Sci., 257, p. 522-532 (1979) CODEN CPMSE; Zur Genauigkeit der Teilchengrossenanalyse durch Sedimentation im Zentrifugalfeld" Alex, Dissertation for PhD in Engineering 15 Jul 1972 Universitat Kalsruhe. Reference may also be had to the following U.S. Pat. Nos. 2,956,434 Donoghue; 3,237,455 Slater; 3,243,106 Atherton, and the following articles; Jones et al "Particle Size Analysis of Inorganic Pigments" Anal. Chim Acta 38, (1967) 143-146; Proc. Soc. Anal. Chem., Particle Size Analysis Group, July 1966 "The ICI--Joyce Loebl Disc Centrifuge", Jones; Powder Technology, 13 (1976), 215-221 "Particle Size Determination of Pigments with a Disc Centrifuge, " Brugger; "Centrifugal Sedimentation, Fraser, Pigments Handbook III-A-d-2-ii, pp. 53-62; "The Accuracy and Precision of the Centrifugal Disc Photosedimentometer Method of Particle Size Analysis", Burt, Powder Technology, 1, (1967) 103-115; "Size Analysis of Organic Pigments using the ICI--Joyce-Loebl Disc Centrifuge", Beresford, Jour. Oil & Color Chem. Assoc. Vol. 50, (1967) 594-614; "The Measurement of Particle Size and its Practical Significance in Vat Dye Quality", Atherton et al, J. Soc. Dyers Colourists 80, p. 521-526 (1964); G. P. Langer, "Sedimentation Analysis of Aqueous Polymer Dispersions with a Disc Centrifuge; " Colloid and Polymer Sci., 257, 522-532 (1979); and Scarlett et al, "The Two Layer Method of Particle Size Analysis", Particle Size Analysis Conference, 1966 pages 242- 267.
From the foregoing patents and literature, it will be seen that the industry method of choice is the Joyce-Loebl buffered line start method for particle size determination. Because of its improved structure and control means the apparatus described and claimed in U.S. Pat. No. 4,311,039 is especially useful in carrying out the buffered line start method.
We have found a new and alternative method for making centrifugal disc photosedimentometric analyses which also allows simplification in the aforesaid improved apparatus as well as the apparatus of U.S. Pat. No. 3,475,968. Power interrupting means utilized in both devices can, if desired, be eliminated. In any event, such power interrupting means are not used in practicing this invention. Moreover, the results obtained according to the new method as will be shown herein, are virtually identical with the results obtained with the apparatus of U.S. Pat. No. 4,311,039 using the buffered line start method described in U.S. Pat. No. 3,475,968 and in much of the prior art mentioned above. Basically, the new method contemplates formation of the spin fluid including a density gradient, externally of the disc and injecting such externally formed spin fluid into the open center of the disc while it is spinning at a predetermined speed.